Thought proper polygraph tests were not so much designed to detect lies as to detect reactions to information that giveaway the subject's involvement in a crime. For instance say they're asking random control questions about items of clothing, then amidst this they show/describe the exact clothes that the victim wore - an innocent person will be oblivious whereas a guilty suspect might be momentarily startled. There's various approaches but none of them are just 'did you kill this man? Were you really at home watching TV?'